A necropsy found that the five-year-old wolf nicknamed Isabelle died last month of a pellet gun wound. That came as a surprise because the gun wound wasn't visible, Park Superintendent Phyllis Green said.

"It is a fluke in some ways, because basically this very small, low velocity pellet managed to go between two ribs and get into an artery and cause damage and kill the animal," she said.

Green said the wolf crossed to the mainland on an ice bridge that formed across Lake Superior.

"She did prove that that ice bridge is a working bridge, hopefully it will be a two-way one at different points in time," Green said.

This winter's ice cover on Lake Superior had raised hopes that a wolf would cross to Isle Royale and rejuvenate the population. There are now only 10 wolves remaining on the island.